It's very close to being natural! Here's another suggestion: Before I had a dog, I couldn't *imagine* my life with *one*. But *now* I have *one*, I find myself never wanting to be without him, even for *a* second."

"Before I had a dog, I couldn't 'imagine' a life with a dog. But when I 'got' a dog, I find myself never wanting to be without him even for one second." It's a little more convoluted than necessary to get the meaning across but it's fine. I would've went with something like: Before owning a dog, I never imagined a life with one. When I finally got one, I find myself wondering; "How did I ever managed to live without one?"

"Before I had a dog, I couldn't image a life with a dog. But when I have a dog, I find myself never wanting to be without him even for one second." After having a dog I couldn't imagine a life without one. When with my dog I find myself never wanting to leave his side, even for one moment. Sounds a bit cheesy, but that should be better. And I changed "before" to "after" for a reason. Before you had a dog, you didnt know anything about owning a dog, but after owning a dog you now have knowledge.